Ground-Based Camera for Exoplanet Imaging

by Paul Gilster on December 29, 2007

How likely is it that we will begin to image extrasolar planets from observatories on the ground? The prospect seems all but certain if we grant a long enough lead time to get certain advanced telescope designs built, but it may happen sooner than we think if the news from the Subaru Telescope is factored in. The instrument, an 8.2 meter optical/infrared telescope, is located at the summit of Mauna Kea (Hawaii), from which vantage it has already produced intriguing results like spin-orbit alignment measurements for the exoplanet TrES-1.

But Subaru astronomer Ryuji Suzuki is ready to go to the next step. Noting the installation of the HiCIAO camera (High Contrast Instrument for the Subaru Next Generation Adaptive Optics), Dr. Suzuki points out that “…the unique instrument was primarily designed for the direct detection of extrasolar planets and disks.” Indeed, the Subaru team is hopeful that they will be the first to directly observe a planet orbiting a star other than our own (we’ll discuss the possible detection of visible light from a planetary atmosphere by another team on Monday).

Image: Not much like the Hawaii most of us imagine, this is the summit of Mauna Kea. Will instruments here and in other locations be able to snare direct images of exoplanets with upgrades to their technologies? Credit: National Astronomical Observatory of Japan.

HiCIAO replaces an older imager that also uses coronagraphic techniques to filter out direct light from a star to make faint objects near it more viewable. But the new system offers contrast 10 to 100 times better than than the old. Its other trump card: A significantly upgraded adaptive optics system that increases the clarity of what Subaru sees by a factor of ten. The latter uses a deformable mirror to effectively remove atmospheric distortion and a new laser guide star system, so that the instrument is not limited in terms of where it can look.

If nothing else, the new instrument should provide impressive results in the study of brown dwarfs, as well as dust disks around nearby stars. But a direct image of a planet — not a brown dwarf — would be a historic first. ‘First light’ use of the new instrument, which occured on December 3, is a reminder of how much ground-based astronomy has improved in the unceasing quest for better seeing conditions, to the point where objects once thought visible only from space are now within its legitimate domain.

though without the detail you provided in your Bad Astronomy post (great job, by the way). However, my impression since then has been that the jury is still out on this one, despite that interesting paper reporting apparent motion in synch with the brown dwarf. Part of the problem involves how 2M1207b formed (I’m sure you’ve run into these objections as well) and whether it shouldn’t be considered a brown dwarf in its own right. I’m playing it cautious, so I’d say it’s a possible planetary image — maybe even a probable one — but I think we’re still looking for an unequivocal exoplanet image about which we can all agree.

I think formation history as the test of planet/brown dwarf status is a furphy especially since cosmogony is a mass of competing theories – Core accertion dominates, but what if all gas planets formed via gravitational instability, which is how stars form? What if only some do? What if brown dwarfs form via instability around larger stars, then get flung loose by tidal encounters? What if Woolfson’s Capture Theory is true, for some systems?

Brown dwarf status should be based purely on something like mass – the deuterium burning limit has always seemed the best way to me. Until we have solid observational reasons to make a distinction, that seems the most physical and least theory dependent difference between intermediate mass objects.

BTW if there’s evidence for enhanced metallicity of an object – a Core Accretion outcome – then perhaps that’s proof positive for ‘planetary status’. A massive Core would be hard to get in a brown dwarf formed via tidal interactions between protostars. But what if a ‘planet’ and ‘brown dwarf’ collided???

Adam, I have to admit I don’t know what a ‘furphy’ is, but it seems evocative enough to explain itself! Like you, I do think formation history as a test of planetary status is questionable, but as long as that question remains, I feel more comfortable seeing 2M1207b as a ‘possible’ planet. Mass does seem like a reasonable way to grade these things, but I think we’ll soon have an exoplanet image that most everyone will agree is the real deal. Doubtless the question of 2M1207b will get resolved somewhere along the way.

Ronald, it does seem as though detecting Earth-like planets may eventually be possible from the Earth’s surface. But what we might achieve with, for example, a sunshade setup like New Worlds, potentially scaling to the point where we can directly image down to continental detail on some planets, seems to demand space as the venue. Obviously, this goes far beyond the basic TPF idea!

Detection is one thing, but when we get our instruments to the scale we want to achieve, I think we surpass what can be done from the surface. Like Ronald, I’d welcome some thoughts on this from other readers as well, particularly with regard to the question of background infrared.

Abstract: We describe the results from a new instrument which combines Lucky Imaging and Adaptive Optics to give the first routine direct diffraction-limited imaging in the visible on a 5m telescope. With fast image selection behind the Palomar AO system we obtained Strehl ratios of 5-20% at 700 nm in a typical range of seeing conditions, with a median Strehl of approximately 12% when 10% of the input frames are selected. At wavelengths around 700 nm the system gave diffraction-limited 35 milliarcsecond FWHMs. At 950 nm the output Strehl ratio was as high as 36% and at 500 nm the FWHM resolution was as small as 42 milliarcseconds, with a low Strehl ratio but resolution improved by factor of ~20 compared to the prevailing seeing. To obtain wider fields we also used multiple Lucky-Imaging guide stars in a configuration similar to a ground layer adaptive optics system. With eight guide stars but very undersampled data we obtained 300 milliarcsecond resolution across a 30X30 arcsec field of view in i’ band.

Abstract: We investigate the limits of ground-based astrometry with adaptive optics using the core of the Galactic globular cluster M5. Adaptive optics systems provide near diffraction-limit imaging with the world’s largest telescopes. The substantial improvement in both resolution and signal-to-noise ratio enables high-precision astrometry from the ground.

We describe the dominant systematic errors that typically limit ground-based differential astrometry, and enumerate observational considerations for mitigating their effects. After implementing these measures, we find that the dominant limitation on astrometric performance in this experiment is caused by tilt anisoplanatism. We then present an optimal estimation technique for measuring the position of one star relative to a grid of reference stars in the face of this correlated random noise source.

Our methodology has the advantage of reducing the astrometric errors as the square root of time and faster than the square root of the number of reference stars — effectively eliminating noise caused by atmospheric tilt to the point that astrometric performance is limited by centering accuracy. Using 50 reference stars we demonstrate single-epoch astrometric precision of ~ 1 mas in 1 second, decreasing to < 100 microarcseconds in 2 minutes of integration time at the Hale 200-inch telescope. We also show that our astrometry is accurate to <~ 100 microarcseconds for observations separated by 2 months.

Finally, we discuss the limits and potential of differential astrometry with current and next generation large aperture telescopes. At this level of accuracy, numerous astrometric applications become accessible, including planet detection, astrometric microlensing signatures, and kinematics of distant Galactic stellar populations.

Abstract: This first Subaru international conference has highlighted the remarkably diverse and significant contributions made using the 8.2m Subaru telescope by both Japanese astronomers and the international community. As such, it serves as a satisfying tribute to the pioneering efforts of Professors Keiichi Kodaira and Sadanori Okamura whose insight and dedication is richly rewarded.

Here I try to summarize the recent impact of wide field science in extragalactic astronomy and cosmology and take a look forward to the key questions we will address in the near future.

Comments: 14 pages, 8 figures; to appear in “Panoramic Views of Galaxy Formation and Evolution”, First Subaru International Conference, Hayama December 2007

Abstract: We present the results of our study of astrometric stability of 200-in Hale (Mt. Palomar) and 10-m Keck II (Mauna Kea) telescopes, both with Adaptive Optics (AO) facilities. A group of nearby visual binaries and multiples was observed in near infrared, relative separations and position angles measured. We have also checked the influence of some systematic effects (e.g. atmospherical refraction, varying plate scale factor) on result and precision of astrometric measurements.

We conclude that in visual binaries astrometrical observations it is possible to achieve much better precision than 1 miliarcsecond, which in many cases allows detection of the astrometrical signal produced by planetary-mass object.

Abstract: There have been numerous reports of anomalies during transits of the planet TrES-1b. Recently, Rabus and coworkers’ analysis of HST observations lead them to claim brightening anomalies during transit might be caused by either a second transiting planet or a cool starspot.

Observations of two consecutive transits are presented here from the University of Arizona’s 61-inch Kuiper Telescope on May 12 and May 15, 2008 UT. A 5.4 +/- 1.7 mmag (0.54 +/- 0.17%) brightening anomaly was detected during the first half of the transit on May 12 and again in the second half of the transit on May 15th.

We conclude that this is a tentative detection of a r greater than or equal to 6 earth radii starspot rotating on the surface of the star. We suggest that all evidence to date suggest TrES-1 has a spotty surface and there is no need to introduce a second transiting planet in this system to explain these anomalies.

We are only able to constrain the rotational period of the star to 40.2 +22.9 -14.6 days, due to previous errors in measuring the alignment of the stellar spin axis with the planetary orbital axis. This is consistent with the previously observed P_obs = 33.2 +22.3 -14.3 day period.

We note that this technique could be applied to other transiting systems for which starspots exist on the star in the transit path of the planet in order to constrain the rotation rate of the star. (abridged)

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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